The present invention relates generally to power metal-oxide-semiconductor field-effect transistors (MOSFETs) and, more particularly, to such devices useful in synchronous rectifier circuit applications and exhibiting low on-resistance, fast switching speed, high voltage capability, and bidirectionality for use in AC circuits.
Power MOSFET devices have a number of advantageous characteristics including high gate impedance, low on-resistance for low forward voltage drop, high withstand voltage capability, and fast switching speeds. Properly gated, they can be employed in synchronous rectifier circuits which previously have employed devices such as conventional PN junction rectifiers, Schottky rectifiers, or bipolar transistor synchronous rectifiers. Power MOSFETs have certain advantages over all of these devices. For example, a PN junction rectifier has a relatively high forward voltage drop (greater than 0.75 volts) and a relatively slow switching speed because upon polarity reversal stored charges must be cleared out before the device will cease conducting. Schottky junction rectifiers substantially eliminate the switching speed problem, but only somewhat alleviate the forward voltage drop problem, which is still greater than approximately 0.5 volts for silicon Schottky-junction devices at high current. Schottky-junction devices generally are lacking in reverse blocking capability in that they have relatively high leakage when reverse biased.
Known power MOSFET structures generally comprise a number of individual unit cells, formed on a single semiconductor wafer, with each device being typically of the order of 300 mils (0.3 inches) square in size and all cells in each device being electrically connected in parallel. Various geometries for the individual unit cells are employed, with an interdigitated comb-like structure being typical. A typical power MOSFET is a double-diffused structure and includes a common drain region of, for example, N conductivity type semiconductor material. Formed within the drain region, preferably by diffusion, is a base region of P conductivity type, and then a source region is formed entirely within the base region. The source region is of N conductivity type, the same as the drain region. At the device surface, the base region exists as a band of P conductivity type semiconductor material between the N conductivity type source and drain regions. A MOSFET gate insulating layer and a conductive gate electrode are disposed over this band. In operation, when gate voltage of the proper polarity (positive for an N-channel MOSFET) is applied to the gate electrode, an electric field is created which extends through the gate insulating layer into the base region to induce a thin N-type conductive layer or channel just under the surface of the base region, providing a continuous, low-resistance N conductivity type conduction channel between the source and drain regions. The actual source and drain terminals comprise metallization on the upper and lower device principal surfaces, a common drain terminal serving all the unit cells. Such device may therefore be considered a vertical current device, although the current flows horizontally in the portion of the conduction channel under control of the gate electrode.
In such power MOSFET structure, the source, base and drain regions correspond respectively to the emitter, base and collector of a parasitic bipolar transistor. As is known, if this parasitic bipolar transistor becomes conductive during operation of the power MOSFET, the blocking voltage and the turn-off speed of the power MOSFET will be substantially degraded. To prevent the parasitic bipolar transistor from conducting during operation of the power MOSFET, it is conventional to electrically interconnect, or "short", the layers comprising the source and base regions by means of an ohmic connection, thus preserving the blocking voltage and turn-off speed of the MOSFET. However, such shorts limit the utility of the resultant devices in some circuits since the device structure inherently includes a parasitic PN junction diode connected directly across the main MOSFET terminals, i.e., the source and drain terminals. For example, in the case of an N-channel enhancement-mode MOSFET structure as summarized above, the P conductivity type base region forms a PN junction with the device drain region. By virtue of the source-to-base short, the P conductivity type bas region is in effect electrically connected to the device source terminal. The N conductivity type drain region is of course connected to the device drain terminal. Consequently, a parasitic PN junction diode exists with its anode connected to the MOSFET source terminal and its cathode connected to the MOSFET drain terminal.
In normal operation of an N channel MOSFET in an electrical circuit, the drain terminal is biased positive with respect to the source terminal. In absence of gate voltage (assuming an enhancement mode MOSFET), the MOSFET device is in a blocking state, and essentially no current flows between the source and drain terminals. When positive gate voltage is applied to turn on the device, an N conductivity type channel is induced and establishes a continuous N conductivity type conductive path through the device between the source and drain terminals. Under these circumstances, the parasitic diode comprising the base and drain regions is of no consequence because it is always reverse biased. The diode cathode (MOSFET drain) is always positive with respect to the diode anode (MOSFET base and source). However, should the polarity of the voltage across the source and drain terminals be reversed, thereby forward biasing the diode comprising the base and drain regions, conduction will occur through the device even in absence of gate voltage (assuming the applied voltage is greater than the approximately 0.6 volts at the "knee" of the diode forward conduction curve, as will be understood by those skilled in the art). In effect, then, the MOSFET device appears as a short circuit for reverse supply voltages. This known characteristic of conventional MOSFET devices limits their ease of application in certain circuits, particularly AC circuits. A MOSFET device which can operate with either supply voltage polarity would potentially be of greater use in actual circuit applications.